Artificial joints are used to replace joints that are inflicted with diseases or severely injured. These artificial joints are unique mechanical-material systems in that they are exposed to biochemical and dynamic environments of the human body and their design is fixed by anatomy and confined by physiological conditions. Critical design elements in these implants include modulus of elasticity, wear resistance, coefficient of friction, corrosion resistance, and biocompatibility with biological tissue and bone. Within these engineering constraints, prosthetic devices must be designed to remain in the body for a lifetime.
Currently, stainless steel, cobalt-chromium-molybdenum, and titaniumaluminum-vanadium alloys are materials used for the prosthesis stem that anchors into the bone. Bone has a modulus of elasticity ranging from 14 to 28 GPa, whereas, the moduli for these alloys range between 110 GPa to 300 GPa resulting in a difference in the moduli of approximately an order of magnitude. Osteoporosis may result from this large mismatch of elasticity between the bone and the implant due to stress shielding. Such bone transformation and degeneration is one of the factors that can promote stem loosening. Following loosening of the stem, the prosthesis ultimately fractures by a fatigue failure mechanism. Metal prostheses, anchored with methyl methacrylate cement, have a useful life of 7 to 10 years. Bond failure necessitates an entire replacement of the prosthesis. Unfortunately, bone resorption due to the presence of the implant limits the number of implant operations to two per patient. As a consequence, joint replacements are restricted to patients over the age of 55. Thus, the medical profession has a need for improved orthopedic devices and biomaterials. A definite need exists for a new material system for extending the expected life of these prosthetic devices for younger patients. If the prosthetic device could be designed to outlive the patient, young adult patients could receive a device without the need for a second replacement operation or fear of being crippled in their later years of life.
With metal implants low concentrations of metallic ions are released with the possibility of allergic reactions. Also, these materials do not promote bone growth into the implant's surface. Bone ingrowth would provide for superior interfacial strength between the bone and biomaterial that is not observed in existing prosthetic device materials.
The biocompatibility of carbon has been demonstrated and has been permitted in the body for many non-load bearing applications. Heart valves have been produced with isotropic pyrolytic carbon and have been successfully implanted in several hundred thousand patients. Orthopedic pins were coated with diamond-like carbon (DLC) and implanted in laboratory animals. The result of the study was that the DLC coating prevented infection of the tissue attached to the pins. Two dimensional carbon/carbon composites have been used as implants in the femurs of rats. The carbon exhibited excellent biocompatibility with the rough surfaces showing a tendency for bone in growth. Prior test results have indicated that carbon/carbon composites provide improved adaptation to bone over titanium implants. Additionally, carbon fibers have provided a degradable scaffold on which ligaments can be regenerated.
Wear is found on the articulation surfaces of current prostheses. Currently, the femoral heads are generally made from either ceramic or metal alloys. The acetabular cups are composed of ultrahigh molecular weight polyethylene (UHMWPE). Wear of the articulation surfaces has occasionally been found to be so intense that the components have had to be replaced. The average clinical wear factor derived from measurements on 25 replaced prostheses was 2.9.times.106 mm.sup.3 /Nm, with results ranging from 0.09.times.106 to 7.2.times.106 mm.sup.3 /Nm. Because penetration rates of the femoral head into the acetabular cup in current forms of total replacement joints are typically in the range of 0.1 to 0.2 mm/year, and overall migrations of the head of 2 or 3 mm is the design limit, the wear lives of implants are in the range of 10 to 30 years. There is increasing concern about the role of wear debris in promoting implant loosening, and consequently there is a need to substantially reduce the volume of wear.
Due to the various limitations of the present implant systems, work has continued on improved implant systems.
One object of the present invention is to produce carbon/carbon composite material with a porosity gradient having about 200 .mu.m diameter pores in one end and fully densified in the opposite end. The porosity will enhance bone ingrowth, thus providing for high interfacial strength between the stem and bone. The fully dense head will provide a smooth substrate for the surface coating. By varying the porosity of the prosthesis material, and by orienting the carbon fibers optimally, the mechanical properties of an implant may be improved.
Another object of the present invention is to form a diamond like carbon coating onto the fully densified carbon/carbon composite material. An ultra-thin layer of DLC on the carbon/carbon composite (head and acetabular cup) should provide extraordinary wear resistance and low coefficient of friction of the mating surfaces.
Still another object of the present invention is to produce a smooth surface on the head portion of the prosthetic device with such a smooth surface resulting from a porosity of less than 10 .mu.m, preferably less than about 5 .mu.m in that portion of the device.
Yet another object of the present invention is to greatly extend the service life of prosthetic devices, e.g., artificial joints, by a materials systems approach employing the union of two or more materials technologies namely the combination of DLC coating technology with carbon/carbon composite material-technology for improved biomedical prostheses. In the case of resultant artificial joints, they should more closely simulate the properties of the actual joint.